The present invention relates to electron, plasma and ion sources for industrial and space propulsion applications. Plasma and ion sources have found important industrial uses particularly in the semiconductor, telecom and optics fields. Another application for plasma and ion sources is as electric engines for space vehicle propulsion. To address these applications a wide variety of sources have been developed. One group of sources can be described as point projection sources. These plasma or ion sources typically include a discharge cavity where plasma is generated and a means to expel or accelerate the plasma or ions from this cavity into a process chamber. Several of these sources implement magnetic fields to control electron motion for improved source performance. Generally, these plasma and ion sources also have at least one electron source to create the plasma in the discharge cavity. Electrons are used to create the plasma because they can readily absorb energy from an electric field and the energetic electrons are used ionize and heat the process gas or propellant.
The present invention relates to an improved point projection plasma or ion source. Prior art point projection type sources pertaining to the present invention include:
U.S. Pat. No. 3,309,873 discloses a plasma accelerator using Hall currents that implemented a solenoid axial magnetic field between a discharge cavity and the process chamber—in this case space. An arc from a tungsten pin to the anode generated electrons to fuel the plasma discharge. No magnetron sputter electron confinement exists inside the discharge chamber.
U.S. Pat. No. 3,913,320 describes an electron-bombardment ion source implementing a hot filament electron cathode centered inside an ion source discharge cavity. An axial magnetic field impedes electron flow to an annular anode. An aligned screen and grid structure separates the discharge cavity from the process chamber and serves to accelerate ions from the discharge cavity plasma into the process chamber.
U.S. Pat. No. 4,871,918 discloses a hollow anode ion-electron source with a plasma beam emanating from a small hole or slit in an anode plate. There is no magnetron confinement in the discharge chamber. Additionally, the hole or slit in the anode is small relative to the electron gyro radius so electrons are not magnetically impeded from the anode hole walls.
U.S. Pat. No. 4,885,070 discloses a method and apparatus for the application of materials using a hot filament type cathode to generate electrons in a discharge chamber connected to a process chamber by a solenoid electromagnet tube conduit. The inner walls of the conduit tube are connected as the anode. Argon gas flows into the tube near the electron emitter and is ionized by the magnetized electrons.
U.S. Pat. No. 5,126,030 discloses a solenoid magnetic field used to conduct the ion flow from a cathode arc discharge from a cathode target in a discharge chamber to the substrates in a process chamber. No magnetron sputter discharge exists in the discharge chamber.
U.S. Pat. No. 5,656,141 describes an apparatus for coating substrates using a heated electron source in a discharge cavity separated from the process chamber by a solenoid electromagnet conduit. Argon is delivered into the conduit near the cathode emitter. The electrons ionize the argon gas and a plasma cloud in the process chamber is created. The heated cathode is lanthanum hexaboride LaB6. The difficulty with this source, as with all sources implementing thermionic emitting cathodes, is the cathode life is shortened by contact with oxygen gas. Because of this, oxygen is not delivered directly into the source but is fed downstream outside the solenoid magnetic field. In addition to downstream oxygen flow, high argon flows through the source are used to keep oxygen from flowing into the source and contaminating the cathode.
The inability to directly ‘burn’ oxygen creates several limitations and challenges: 1) The argon purge gas adds to the overall pumping load mandating larger and more expensive vacuum pumps. 2) The downstream activation/ionization of the oxygen gas is less efficient than ionizing the process gas inside the plasma source. 3) Uniformity of oxygen plasma density at the substrates is reduced when oxygen is introduced outside the discharge chamber. And 4) Even with the argon purge gas, the expensive cathode requires replacement at accelerated intervals when oxygen is used in the process.
U.S. Pat. No. 5,855,745 describes an end hall type ion source centered in a planar magnetron cathode. The planar magnetron is used as the electron source for the ion source and to neutralize the ion beam. In this configuration independent magnetic circuits are used—a racetrack magnetron circuit for the planar magnetron and an axial field for the end hall ion source. The planar magnetron is directly facing the substrates and is not enclosed in a separate discharge chamber.
U.S. Pat. No. 6,323,586 describes a closed drift hollow cathode as an improved electron source. There is no magnetron confinement inside the discharge chamber and electrons are not magnetically impeded from reaching the chamber side walls nor are they magnetically conducted out of the discharge chamber to the process chamber.
An improved plasma source would be able to directly ‘burn’ oxygen gas in the plasma source without requiring an inert purge gas. It would also not require expensive filaments or thermionic cathode materials that are fragile and require frequent replacement. The improved plasma source would also not have hot cathodes that introduce a thermal load on the substrates.